1. Field of the Invention
The present invention relates to image capturing apparatuses, imaging circuits, and image capturing methods for capturing images with solid state imaging devices. In particular, the present invention relates to an image capturing apparatus that temporarily stores image signals resulting from an image capturing operation and processes the image signals, an imaging circuit suitable for such a configuration, and an image capturing method.
2. Description of the Related Art
Recently, the use of imaging capturing apparatuses, such as digital still cameras and digital video cameras, capable of capturing images with solid state imaging devices and of storing the captured images as digital data has been widespread. In such an image capturing apparatus, the number of pixels of imaging devices is increasing, the functions of the apparatus are more advanced, and the performance of the apparatus is becoming higher. In particular, an increase in the number of pixels of imaging devices leads to an increase in a load for processing imaging signals. It is desired that even such an image capturing apparatus can process the imaging signals at high speed so as not to put a stress on operations.
FIG. 11 is a block diagram showing an example of a configuration of a known image capturing apparatus.
The known image capturing apparatus shown in FIG. 11 has imaging devices 81, an AFE (analog front end) circuit 82, a digital image processing circuit 83, an SDRAM (synchronous dynamic random access memory) 84, a ROM (read only memory) 85, and a storage device 86. In addition, the digital image processing circuit 83 includes a camera signal preprocessing section 91, a camera signal processing section 92, a resolution converting section 93, a JPEG (joint photographic experts group) engine 94, a CPU (central processing unit) 95, a video output encoder 96, and an SDRAM controller 97, which are connected to each other through an internal bus 98.
In the image capturing apparatus having such a configuration, imaging signals of an image captured by the imaging devices 81 are sequentially supplied to the AFE circuit 82. After undergoing a CDS (correlated double sampling) operation and an AGC (auto gain control) operation, the imaging signals are digitalized and supplied to the digital image processing circuit 83. The camera signal preprocessing section 91 performs operations, such as defect pixel correction and shading correction, on the supplied image signal to generates RAW data, and writes the RAW data in the SDRAM 84 through the SDRAM controller 97.
The camera signal processing section 92 reads out the RAW data from the SDRAM 84 through the SDRAM controller 97. After performing the various detection operations and an image quality correction operation (i.e., camera signal processing operations) on the RAW data, the camera signal processing section 92 converts the RAW data into a luminance signal (Y) and color-difference signals (R-Y and B-Y), and outputs the luminance signal and the color-difference signals. The resolution converting section 93 converts the resolution of the image data output from the camera signal processing section 92, if necessary.
The video output encoder 96 converts the image data, whose resolution has been converted into a resolution suitable for displaying by the resolution converting section 93, into an image signal for displaying an image on a monitor, and outputs the converted image signal to a monitor (not shown) or a video output terminal 96a. This allows a camera-through image to be displayed. The JPEG engine 94 compresses and encodes the image data supplied from the camera signal processing section 92 or the resolution converting section 93 according to a JPEG standard, and temporarily stores the encoded image data in the SDRAM 84. The CPU 95 records the JPEG encoded data stored in the SDRAM 84 on the storage device 86.
The CPU 95 controls operations performed in the entire image capturing apparatus in an integrated manner. The ROM 85 stores programs executed by the CPU 95 and data necessary for the operations.
In the above, the image capturing apparatus that records captured images as JPEG data has been described. However, an image capturing apparatus having a function to directly record RAW data, not having undergone the camera signal processing operations, on a recording medium is also realized. For example, there is an image capturing apparatus that has a function to compress RAW data according to a reversible compression method that utilizes a Huffman table and to record the compressed data, and that optimizes the Huffman table for each color channel (for example, see Japanese Unexamined Patent Application Publication No. 2004-40300 (Paragraph Nos. [0019] to [0028], FIG. 2)). In addition, there is also an image capturing apparatus that bypasses an RAW data interpolation processing section used in a normal compression mode when the mode is set in a RAW compression mode for compressing and recording RAW data (for example, see Japanese Unexamined Patent Application Publication No. 2003-125209 (Paragraph Nos. [0027] to [0037], FIG. 1)).
As shown in FIG. 11, a general image capturing apparatus temporarily stores RAW data obtained from imaging devices in an image memory such as an SDRAM, and then reads out the RAW data and performs a camera signal processing operation on the read out RAW data. For example, in a case where an apparatus completes capturing of a frame with a plurality fields, such as a case where the apparatus uses interlaced scanning imaging devices, frame data is generated after data of each field is stored in a memory. In addition, an apparatus may have a processing system that partially processes data (for example, several lines in a vertical direction in a rectangular shape) of a entire screen image using only a delay line equivalent to a fraction of 1H (horizontal synchronization interval) to suppress the size of a line memory included in the camera signal processing section. In such a case, it is also necessary to store the data for the entire screen image in the memory at least before the processing.
When RAW data is written in and read out from a memory, the data for the entire screen image is transferred over an interval bus. Thus, most of the bus band used at the time of image capturing is occupied by this transfer. In particular, as the number of pixels of imaging devices and the size of the RAW data increase, a data transfer load increases, which undesirably requires a longer time for writing and reading out data in and from a memory. Accordingly, an attempt to decrease an amount of time for a recording operation requires an increase in the bus band by setting a transmission frequency higher or the like, which undesirably increases a cost of the apparatus. Additionally, an increase in the number of pixels undesirably leads to an increase in a capacity of a memory storing the RAW data.
On the other hand, it is also considered to compress RAW data before transferring the RAW data over the internal bus. If a variable-length coding method is employed as the compression method, a bus band necessary for the transfer may not be kept constant, which undesirably complicates the processing and prevents an advantage of reducing the bus band from being provided.
In the above-cited Japanese Unexamined Patent Application Publication No. 2004-40300, the RAW data is compressed according a variable-length coding method. In addition, in both the above-cited Japanese Unexamined Patent Application Publication Nos. 2004-40300 and 2003-125209, the RAW data is not compressed to reduce the internal bus band.